1. Field of the Invention
Embodiments of the present invention relate generally to methods and devices for depositing viscous materials onto a printed wiring board. In one aspect, the present invention relates to methods and devices for compressing viscous materials, such as solder paste, through openings in a perforated substrate, such as a patterned screen or stencil.
2. Description of Related Art
Surface Mount Technology (SMT) involves placing circuit components onto circuit paths embedded on the upper surface of a printed wiring board and then soldering the components in place by a process called "reflow soldering". Before the circuit component is placed on the printed wiring board, however, it is desirable to apply solder paste to the area on the printed wiring board where the component is to be soldered into place.
Conventional methods do exist to deposit ("print") solder paste onto desired areas of a printed wiring board by forcing the paste through openings in a substrate (e.g., a stencil) placed in intimate contact with the printed wiring board.
U.S. Pat. No. 4,622,239 describes such a method and device for dispensing viscous materials. The method includes forcing a viscous material from a housing through an opening and depositing it onto a stencil between a pair of flexible members (parallel squeegee blades) which depend from the housing on either side of the opening and are in contact with the stencil. The ends of the flexible members are not connected and remain open ended. The viscous material, accordingly, is not contained within an enclosed area when it is deposited on the surface of the stencil. Movement of the housing and the flexible members horizontally across the stencil causes the trailing flexible member to force the viscous material through the openings in the stencil. U.S Pat. No. 4,720,402 describes a similar method and device except that the leading flexible member is raised off of the stencil during movement of the housing.
U.S. Pat. Nos. 5,133,120 and 5,191,709 describe methods for filling through-holes of a printed wiring board via a mask with pressurized conductive filler material by means of a nozzle assembly unit having a nozzle tip member. The nozzle tip member, however, is designed only to dispense the pressurized conductive filler material through the mask to a single through-hole. The nozzle tip member then "scans" the printed wiring board for a second through-hole to fill. The nozzle tip member has a blunt end section which rests on the mask and a circular exit, the diameter of which may be increased or decreased by changing the nozzle tip member. The nozzle tip member dispenses the filler material without controlling unwanted flow of "excessive" filler material back through the stencil. Additionally, the nozzle tip member does not define a contained environment where "compression" of the filler material takes place through the mask followed by the immediate shearing off of the filler material within that contained environment from the surface of the stencil. In fact, the nozzle tip member itself provides no effective means for shearing off filler material from the top of the stencil, rather, after the through hole is filled and filler material "backs up" through the stencil, the nozzle tip member moves forward whereupon the "excessive" filler material is then wiped off by a separate, single, flexible squeegee member which is designed for unidirectional use only.
Unfortunately, these conventional efforts do not provide a contained environment for compression of viscous material through holes in a stencil and shearing of viscous material within the contained environment from the upper surface of the stencil. Reliance upon squeegee movement to force the viscous material, such as solder paste, through the stencil openings can lead to damage and eventual failure of both the squeegee blades and the stencil due to repeated friction. Since conventional efforts do not provide a contained environment in which compression and shearing is accomplished, waste of the viscous material is frequently encountered.
Conventional efforts, therefore, (1) fail to maximize the efficiency of printing solder paste onto a desired area of a printed wiring board and (2) fail to minimize waste of the solder paste during the printing process. A need therefore exists to develop a method for printing solder paste onto a printed wiring board and a device suitable for use therewith which overcomes the deficiencies of the conventional efforts.
Other methods which utilize a pneumatically driven piston, or a direct injection or application of air or other gasses, to force viscous material from a syringe, have several drawbacks. For example, air or other gases which are used to directly expel the paste or viscous material from the container, or which are used to selectively move a piston, oftentimes cause undesirable voids in material. Moreover, in order to selectively expel the entire contained quantity of the viscous material from the syringe, the stroke of the pneumatically driven piston must be substantially as long as the syringe. The use of relatively small and inexpensive conventional pistons therefore requires the concomitant use of many short syringes which must be frequently replaced and/or re-filled, thereby undesirably increasing production time and concomitantly decreasing production efficiency. The use of longer more expensive pistons allows for the use of larger syringes containing more viscous material and requiring less frequent replacement and/or re-filling. However, these large pistons are relatively heavy, thereby requiring costly and undesirable structural modifications to the compression head, are difficult to package, and/or are difficult and/or awkward to replace.
The present invention is directed at eliminating the need for a pneumatically driven piston while allowing for the selective distribution of a relatively large amount of viscous material.